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This technique borrows its name from the Solo / Map deduction in which you find a path of vertices in your graph each of which has two possible values, such that a choice for one end vertex of the chain forces everything along it. In Dominosa, an approximate analogue is a path of squares each of which has only two possible domino placements remaining, and it has the extra-useful property that it's bidirectional - once you've identified such a path, either all the odd domino placements along it must be right, or all the even ones. So if you can find an inconsistency in either set, you can rule out the whole lot and settle on the other set. Having done that basic analysis (which turns out to be surprisingly easy with an edsf to help), there are multiple ways you can actually rule out one of the same-parity chains. One is if the same domino would have to appear twice in it; another is if the set of dominoes that the chain would place would rule out all the choices for some completely different square. There are probably others too, but those are the ones I've implemented.
This is the README accompanying the source code to Simon Tatham's puzzle collection. The collection's web site is at <https://www.chiark.greenend.org.uk/~sgtatham/puzzles/>. If you've obtained the source code by downloading a .tar.gz archive from the Puzzles web site, you should find several Makefiles in the source code. However, if you've checked the source code out from the Puzzles git repository, you won't find the Makefiles: they're automatically generated by `mkfiles.pl', so run that to create them. The Makefiles include: - `Makefile.am', together with the static `configure.ac', is intended as input to automake. Run `mkauto.sh' to turn these into a configure script and Makefile.in, after which you can then run `./configure' to create an actual Unix Makefile. - `Makefile.vc' should work under MS Visual C++ on Windows. Run 'nmake /f Makefile.vc' in a Visual Studio command prompt. - `Makefile.cyg' should work under Cygwin / MinGW. With appropriate tweaks and setting of TOOLPATH, it should work for both compiling on Windows and cross-compiling on Unix. - `Makefile.osx' should work under Mac OS X, provided the Xcode tools are installed. It builds a single monolithic OS X application capable of running any of the puzzles, or even more than one of them at a time. - `Makefile.wce' should work under MS eMbedded Visual C++ on Windows and the Pocket PC SDK; it builds Pocket PC binaries. Many of these Makefiles build a program called `nullgame' in addition to the actual game binaries. This program doesn't do anything; it's just a template for people to start from when adding a new game to the collection, and it's compiled every time to ensure that it _does_ compile and link successfully (because otherwise it wouldn't be much use as a template). Once it's built, you can run it if you really want to (but it's very boring), and then you should ignore it. DO NOT EDIT THE MAKEFILES DIRECTLY, if you plan to send any changes back to the maintainer. The makefiles are generated automatically by the Perl script `mkfiles.pl' from the file `Recipe' and the various .R files. If you need to change the makefiles as part of a patch, you should change Recipe, *.R, and/or mkfiles.pl. The manual is provided in Windows Help format for the Windows build; in text format for anyone who needs it; and in HTML for the Mac OS X application and for the web site. It is generated from a Halibut source file (puzzles.but), which is the preferred form for modification. To generate the manual in other formats, rebuild it, or learn about Halibut, visit the Halibut website at <https://www.chiark.greenend.org.uk/~sgtatham/halibut/>.